A hybrid vehicle may selectively enter and exit a regeneration mode or regenerative braking where the vehicle's kinetic energy is converted to electrical energy and stored for later use. The vehicle may enter regeneration mode during times when driver demand is low, such as when the hybrid vehicle is traveling down a road that has a negative grade. An electric machine provides a negative torque to the hybrid vehicles powertrain during regeneration. The negative torque helps to provide vehicle braking, but vehicle braking may also be provided by friction brakes. If negative torque provided by the electric machine is not applied to the vehicle's wheels due to transmission degradation, the vehicle may not decelerate at a desired rate.
One or more transmission components or control commands may temporarily provide less than desired transmission operation during regeneration. For example, a transfer function that describes clutch torque capacity versus clutch pressure may over estimate clutch torque capacity during a shift. Consequently, the clutch may slip when it is applied; thereby causing the transmission's input shaft speed to be reduced more than is desired. In other examples, the clutch transfer capacity may be reduced due to a partial line blockage or degraded control solenoid. Consequently, the clutch may transfer a fraction of regeneration torque from the wheels to the electric machine. In still other examples, the torque transfer capacity of a clutch may be completely degraded so that only a small amount of regenerative torque is transferred from vehicle wheels to the electric machine. It would be desirable to recover from the above mentioned conditions of transmission degradation in a way that continues to provide regenerative braking and that reduces the possibility of degraded vehicle drivability.
The inventors herein have recognized the above-mentioned issues and have developed a powertrain operating method, comprising: predicting a transmission input shaft speed from a transmission output shaft speed; adjusting a regeneration torque of an electric machine coupled to the transmission in response to an actual transmission input shaft speed minus the predicted transmission input shaft speed and a type of transmission degradation.
By reducing a regeneration torque toward a value of zero in response to transmission input shaft speed minus a predicted transmission input shaft speed and a characterization of a type of transmission degradation, it may be possible to provide the technical result of gracefully recovering from a condition of transmission degradation during regeneration. In some examples, the condition of transmission degradation may be during a downshift or while a transmission clutch is commanded closed. Further, it may be possible to reduce subsequent driveline torque disturbances by avoiding commanding clutches that may be classified as degraded.
The present description may provide several advantages. Specifically, the approach may provide improved recovery from conditions of transmission degradation. In addition, the approach may selectively apply different mitigating techniques that may be more suitable for types of degradation encountered. Further, the approach may reduce a regenerative torque so that at least some regenerative braking may be provided during some conditions of transmission degradation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.